1. Field of the Invention
This invention relates to composite hollow fiber type selective separation membranes exhibiting excellent selective separation properties and permeation rate, processes for the preparation thereof, and their use.
2. Description of the Prior Art
In recent years, a number of composite separation membranes have been developed in which a thin film of a polymer having highly selective separation permeability to specific substances is uniformly formed on the surface of a porous body. In addition, many processes have been developed for the purpose of fabricating separation membranes exhibiting excellent selective separation properties and permeability at high flow rates for use as gas separation membranes, such as oxygen enrichment membranes, reverse osmosis membranes, membrane-type artificial lungs, and the like.
Conventional composite membranes exhibit an extremely thick interpenetrating layer because the component responsible for selective separation has penetrated into the minute pores on the surface of the porous support. Because of such a thick interpenetrating layer, the permeation flow rate is extremely low.
For example, in Japanese Patent Application Laid-open No. 86684/1978 (British Patent Application Laid-open No. 7707583), there is disclosed a composite separation membrane for gas separation in which the outer surface of an aromatic polysulfone porous hollow fiber is coated with a thin film of silicone rubber by dipping the porous hollow fiber of the aromatic polysulfone into a solution of a polydimethylsiloxane and a curing agent dissolved in n-heptane and thereafter heating and drying to cure and polymerize the polydimethylsiloxane to convert it to the silicone rubber. However, in this composite separation membrane, the coated silicone rubber penetrates extremely deeply into the minute pores on the porous base surface, and therefore the permeation performance, for example, when used as an oxygen enrichment membrane, is extremely small, e.g., about 10.sup.5 ml/24 HRS.multidot.m.sup.2 .multidot.(kg/cm.sup.2), which requires an improvement of the permeation rate by about 100 times in order to be useful in active practice.
Also, since the component responsible for selective separation easily penetrates into the minute pores, it is extremely difficult to completely cover the base surface having the minute pores. As a result, pinholes are developed in the region where the covering is inadequate. Composite separation membranes having such pinholes fail to provide the separation performance otherwise inherent in the selective separation component.
Japanese Patent Publication No. 25451/1981 discloses an improved composite membrane structure having no interpenetrating layer within the minute pores of the porous support. The improved composite membrane is obtained by impregnating the minute pores of the porous support with a liquid to temporarily make the surface of the porous support non-porous and flat, then forming a selective separation layer on its surface, and thereafter removing the liquid impregnated within the minute pores. However, since the surface of the support is inevitably wetted with the impregnating liquid, the adhesion between the porous membrane base and the selective separation layer of the resulting composite membrane is often inadequate and therefore, the membrane is subject to the risk that the selective separation layer can peel off from the base during handling and result in damage to the selective separation layer.
Further, Japanese Patent Publication No. 17589/1973 discloses a two-layered composite selective permeation membrane structure which comprises a uniform porous base and a thin, selectively permeable polymer film formed thereon without an interpenetrating layer by use of a fine powder pore-making agent. However, in this selective permeation membrane, since the pore size of the uniform porous base is equal between the outer surface and the inner surface thereof and is extremly small, e.g., about 100 .ANG., as is clear from the model in FIG. 2 of Japanese Patent Publication No. 17589/1973, the fluid permeation resistance of the porous base per se is extremely high, and thus even though the fluid permeability of the selectively permeable polymer film per se is adequately high by forming a thin selectively permeable polymer film on the porous base without an interpenetrating layer, the fluid permeability of the two-layered composite membrane as a whole is quite small and also is unsatisfactory in practical use in view of the limited permeation rate.
Composite separation membranes may be roughly classified into two types: one being a film membrane type composite separation membrane is which a selective separation layer is formed on a film membrane type porous support, but such film membrane type composite separation membranes characteristically have a fundamental problem in that it is impossible to have a large effective membrane surface area per unit volume when such membranes are formed into a module, and thus the resulting separation system has to be quite large. A solution to the above problem was the development of composite hollow fiber type separation membranes in which a selective separation layer is formed on the surface of a hollow fiber type membrane porous support, in which case, it is presumed that the effective membrane surface area per unit volume upon module formation can be made larger by 10-100 times, and therefore, it is apparent that this type of membrane provides an excellent advantage enabling the development of a correspondingly small-sized compact separation system.